Courtesy Reuters
Chinese People's Liberation Army Navy recruits march during a parade to mark the end of a semester at a military base of the North Sea Fleet, in Qingdao, Shandong province, December 5, 2013.

China’s recent announcement of an Air Defense Identification Zone in the East China Sea has generated a great deal of confusion and alarm. Much of that is a function of the fact that few know what an ADIZ is, what it is for, and why it matters -- including, apparently, the Chinese government and military.

An ADIZ is a publicly defined area extending beyond national territory in which unidentified aircraft are liable to be interrogated and, if necessary, intercepted for identification before they cross into sovereign airspace. The concept is a product of the Cold War: in the 1950s, the United States declared the world’s first ADIZs to reduce the risk of a surprise attack from the Soviet Union. Today, the United States has five zones (East Coast, West Coast, Alaska, Hawaii, and Guam) and operates two more jointly with Canada. Other countries that maintain ADIZs include India, Japan, Norway, Pakistan, South Korea, Taiwan, and the United Kingdom.

In addition to providing an added measure of security, an ADIZ can help reduce the risk of midair collisions, combat illicit drug flows, facilitate search-and-rescue missions, and reduce the need for fighter jet sorties for purposes of visual inspection. This last point is the most important: ADIZs can increase transparency, predictability, and strategic stability by reducing uncertainty on both sides about when, where, and how aerial interceptions might take place. In 1960, for example, the Soviet Union had no clearly established air defense identification zones and procedures, and the resulting confusion led to a U.S. reconnaissance aircraft being shot down over international waters.

There are no international agreements governing any aspect of an ADIZ. States are neither explicitly authorized to establish them nor are they explicitly prohibited from doing so. ADIZs usually extend into what is universally acknowledged to be international airspace, even by the countries that maintain them, and in no way confer any sovereign rights. Off southern California, for example, a U.S. ADIZ stretches more than 400 miles out